Scientists try to identify a way to form the Moon, and find two.

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The Moon is a bit of an enigma. In some ways, it's nothing like Earth, as its minerals contain few volatile chemicals and it has a relatively tiny core. But in other ways, it's nearly our twin, with many elements having isotopic signatures that are almost identical.

Currently, our best model for Moon formation involves having a Mars-sized object smack in to the early Earth. This could create a Moon that has some similarities to the Earth, but ends up with most of the iron from the impact being deposited in the Earth's core. The only problem with this is that anything as big as Mars probably originated from elsewhere in the Solar System, and thus would have a very distinct isotope ratio.

Today, Science is releasing two papers that take very different routes to tackling this problem. One models what would happen if, instead of a large size difference between the Earth and its impactor, the two bodies were of roughly equal size. Another models two differently sized bodies colliding, but assumes the proto-Earth was spinning much faster than it is now, with "days" on the order of 2.5 hours long. And, in a dilemma that may interest planetary scientists, both models produce the sort of distribution of materials we currently see.

Models of Solar System formation suggest that its rocky planets were built sequentially, with planetesimals condensing from the dust and debris, and then merging to form protoplanets. Over time, these protoplanets underwent a series of collisions and mergers, building planets like Venus and the Earth. (A few objects, like Mars and Vesta, may have sat out most of these later mergers, leaving much smaller objects behind.)

A collision of this sort could help explain some puzzling aspects of the Moon. If the debris left behind were dispersed enough, some of it could condense into a separate object, explaining how two large bodies could end up in such close proximity. And heavy elements would preferentially end up in the larger one, explaining the Moon's small core.

The simulations that show how these collisions would work involve smacking together two spheres full of particles, with each particle having a distinct identity and location—heavy metal particles in the core, silicate rocks in the crust, etc. By tracing these particles through the collision, it's possible to follow the iron from the impactor, and watch it drop into the Earth's core, and so on. The problem is that they also show that the Moon should end up with a crust that's largely composed of material from the impactor. And that's tough to square with the fact that the different forms of many elements, called isotopes, vary with a body's location in the Solar System.

So, unless the impactor started right next door, we'd expect that the isotope ratios of the material it brought wouldn't look like the ones on Earth. And, as we look more carefully at the material in the Moon, that just doesn't seem to be the case.

One of the new papers, from the Southwest Research Institute's Robin Canup, takes a look at what would happen if two large bodies combined. Typically, the Earth is modeled as being nearly its current size, the product of multiple planetoid mergers; its impactor as being about a tenth of its size. Canup ran a series of models in which the two were much closer to equal size, starting with an impactor that was half the mass and moving up to one that was about 90 percent. Models where the impactor was about 80 percent of the size of the pre-Earth created debris disks that could form a Moon; both it and the resulting Earth ended up with very similar material in their crusts.

A second paper, from Sarah Stewart of Harvard and Matija Ćuk (now with the SETI institute), went about things completely differently. They focused on the fact that all the collisions involved in building a planet are expected to leave it spinning very rapidly. So they modeled a normal Mars-sized impactor, but had the Earth spinning really fast, with days lasting anywhere from 2.3 to 2.7 hours. These collisions produce a debris disk that was composed primarily of material from the pre-Earth's mantle, which would explain the Moon's present similarity to the Earth.

An example of one of the runs of this model is seen here. The smash leaves the impactor's matter evenly distributed, and a sufficient amount of debris far enough from the Earth to form a separate body.

The problem with both of these models is that they leave the Earth-Moon system spinning very rapidly. Since the angular momentum of the system has to be conserved, we need some way of getting rid of some of this spin. Tidal forces can do some of that, but the authors of the second paper go on to show that there's a resonance between the lunar orbit and the system's orbit around the Sun. This effectively moves some of the angular momentum out of the Earth-Moon system, and into the system's orbit around the Sun. Combined with tidal forces, this can put a strong enough brake on the system.

Which model will win out? Right now, neither paper really addresses the others' model, so it's a bit hard to say. It's possible that, as the details are filled in, one or the other model will provide a better match to the data we already have. Or one of them could identify data we don't have yet. The ongoing GRAIL mission, which is mapping the density of the Moon's crust, may also provide some further information that will help us understand the Moon's formation.

Promoted Comments

This comment got me curious so I did some rough calculations. Using a perfectly squerical earth of radius 6371km I get for a current rotation around the axis of 24 hours the centripetal acceleration is ~0.03m/s^2 which means there is a fairly negligible difference in how heavy you feel at the poles vs. the equator. However for a 2.5 hour rotation the centripetal acceleration becomes ~3.1m/s^2 which is a big chunk off of gravity at 9.8m/s^2.

If this is actually what early earth was doing it must have been extremely fat around the middle.

This effectively moves some of the angular momentum out of the Earth-Moon system, and into the system's orbit around the Sun. Combined with tidal forces, this can put a strong enough break on the system.

I recall the earth is slowing because of the tidal forces and the moon will eventually go into tidal lock. Can the slowing of rotation be calculated backwards to get an idea of length of a day back a few billion years ago?

So are they still running a simulation like the one in the video above to see if another body is formed from the material in the moon-forming disk that will match the moon's properties or are they just reasonably satisfied that such would occur given what we already know?

I recall the earth is slowing because of the tidal forces and the moon will eventually go into tidal lock. Can the slowing of rotation be calculated backwards to get an idea of length of a day back a few billion years ago?

Actually the moon is already tidally locked. That's why we always see the same side.

I recall the earth is slowing because of the tidal forces and the moon will eventually go into tidal lock. Can the slowing of rotation be calculated backwards to get an idea of length of a day back a few billion years ago?

Actually the moon is already tidally locked. That's why we always see the same side.

I think he's trying to say that the earth will be tidally locked relative to the moon (standing on the moon, you'd see the same face of the earth) in the distant future - like Pluto and Charon. If I recall correctly though, this will never actually happen because the sun will expand in its red giant phase to engulf the earth first.

I recall the earth is slowing because of the tidal forces and the moon will eventually go into tidal lock. Can the slowing of rotation be calculated backwards to get an idea of length of a day back a few billion years ago?

Those calculations have already been done, and the event occurred fairly early in Earth's existence.

The moon would eventually tidal lock, but it's moving away from the Earth and would eventually escape. However, the sun will die long before either happens.

OOOORRRR.... The moon was towed into place by the same beings that placed us here. How do we take into account some of the ancient texts and pre-history that recall the night sky before the moon was there?

It's just a theory as good as theirs, minus the animation.

Great thing about theories with scientists, is that any theory can hold water until it is proven otherwise. All they have is a flimsy animation that is incomplete. I can point to actual texts or even find some tribesman that has been charged with passing such knowledge down. Even though the latter is hearsay, it at least gives us another option to ponder.

Except the two models are based in part on known verifiable data. Your creationist conjecture isn't. And no, a valid scientific theory has to align with reality. A good scientific theory is one that can create testable hypotheses. Your conjecture can't even be considered a valid hypothesis unless you can figure out a falsifiable method for testing it.

OOOORRRR.... The moon was towed into place by the same beings that placed us here. How do we take into account some of the ancient texts and pre-history that recall the night sky before the moon was there?

It's just a theory as good as theirs, minus the animation.

Great thing about theories with scientists, is that any theory can hold water until it is proven otherwise. All they have is a flimsy animation that is incomplete. I can point to actual texts or even find some tribesman that has been charged with passing such knowledge down. Even though the latter is hearsay, it at least gives us another option to ponder.

The summary is, a good explanation takes into account all of the known facts. A good explanation cannot have its components vary - every part of the explanation is important to the outcome.

While there is still much that is not known about the origin of the Earth and the Moon, both of these conflicting explanations explain in detail a number of key facts, such as the isotope signatures, the heavy iron-based Earth core, and the similar materials in the crusts.

The "towing of the moon into place" does not explain these facts about isotope signatures, material composition, and others, and that's why we're more likely to believe these scientific theories more than yours.

Aarrgghh!! Why does that animation stop before the money shot? There's no moon!!!!

The animation is on the scale of about 24 hours of simulation time to 1 minute of real time. So a year's worth of simulation time would take about 6 hours to run, meaning you could model about 4 years per day, or not quite 1500 years per year.

Per Wikipedia, the time for the moon to coalesce could have taken anywhere from a month to a century; either way, I think you'd run out of patience.

Okay, so if two large objects with different isotope ratios merge into a single object, and then that single object splashes off some of itself to form another object, which has the same composition as the object it splashed from, why is this not computing?

where he analyzes a Nature paper that suggests that the original collision with the Earth created not one but two objects, which later collisioned themselves and produced the different characteristics of each of the Moon sides.

I recall the earth is slowing because of the tidal forces and the moon will eventually go into tidal lock. Can the slowing of rotation be calculated backwards to get an idea of length of a day back a few billion years ago?

We do have records of it going back 400 million years or so in daily growth layers from corals.Google-fu failing... long story short, ~400 million years ago there were about 400 days in the year.

Aarrgghh!! Why does that animation stop before the money shot? There's no moon!!!!

The animation is on the scale of about 24 hours of simulation time to 1 minute of real time. So a year's worth of simulation time would take about 6 hours to run, meaning you could model about 4 years per day, or not quite 1500 years per year.

Per Wikipedia, the time for the moon to coalesce could have taken anywhere from a month to a century; either way, I think you'd run out of patience.

It's an animation. We don't have to watch it all, they can skip to one frame per year (or decade, century, etc) of the simulation and then drop the rate back when something interesting starts to happen.

Okay, so if two large objects with different isotope ratios merge into a single object, and then that single object splashes off some of itself to form another object, which has the same composition as the object it splashed from, why is this not computing?

Apologies for my lack of understanding that a problem exists.

Under the old models, more of the smaller object splashes off than the larger object. These two models have found two different scenarios where the same ratio of materials splash off from each object.

They didn't know all this stuff. What you're describing is a modern misinterpretation of ancient mythology, and it's in stark contrast to actual observations (as well as legit readings of those myths and histories). This idea is wrong on the history, it's wrong on the mythology, and it's wrong on the astronomy.

Aarrgghh!! Why does that animation stop before the money shot? There's no moon!!!!

The animation is on the scale of about 24 hours of simulation time to 1 minute of real time. So a year's worth of simulation time would take about 6 hours to run, meaning you could model about 4 years per day, or not quite 1500 years per year.

Per Wikipedia, the time for the moon to coalesce could have taken anywhere from a month to a century; either way, I think you'd run out of patience.

It's an animation. We don't have to watch it all, they can skip to one frame per year (or decade, century, etc) of the simulation and then drop the rate back when something interesting starts to happen.

I don't think its the time it would take to watch the animation that was the issue, rather the time it would take to run the simulation for that long. I wouldn't be surprised if that 24hr animation took a few days/weeks on a small cluster of machines. Even if you could run it at 1day/hr/cpu, you'd probably need a century or so of simulation time to get a reasonably compete moon. (Or a few days on a Top10 super computer)

"The only problem with this is that anything as big as Mars probably originated from elsewhere in the Solar System, and thus would have a very distinct isotope ratio."

But the classical Theia model (giant impact hypothesis) assumes that the (eventually Mars-sized) impactor started accumulating mass at Earth's lagrange point (L4 or L5), but its orbit became unstable around the time its mass grew big enough. Therefore, as it started from the same part of the disk around the sun, why would it need to have such a different isotope ratio?

So, unless the impactor started right next door, we'd expect that the isotope ratios of the material it brought wouldn't look like the ones on Earth.

Um... the impactor (known as "Theia" -- also sometimes referred to as "Orpheus") is indeed supposed to have originated as our closest neighbor between the orbit of Earth and Mars, or possibly in an almost identical orbit as the Earth, kind of like the movie Journey to the far Side of the Sun.

If the impactor didn't have a similar orbit in the same ecliptic plain when it impacted us, it would have knocked us into a new orbit.

"The only problem with this is that anything as big as Mars probably originated from elsewhere in the Solar System, and thus would have a very distinct isotope ratio."

But the classical Theia model (giant impact hypothesis) assumes that the (eventually Mars-sized) impactor started accumulating mass at Earth's lagrange point (L4 or L5), but its orbit became unstable around the time its mass grew big enough. Therefore, as it started from the same part of the disk around the sun, why would it need to have such a different isotope ratio?

Yeah, I had the same question. It is of course by no means established that this is so, but I seem to recall a paper on the orbital dynamics of this solution which preferentially produces collisions with the right characteristics (there is a fairly narrow keyhole in the collision parameter space under which you end up with a Moon at all, it has to be a glancing blow that is just right). An object falling out of L4/5 will follow a sort of 'horseshoe' orbital resonance with Earth (there is at least one asteroid doing this now). Eventually the object may start passing the Earth, at which point there is a fairly decent chance of an oblique impact.

Formation at L4/5 would make the isotope ratios of Thea pretty much identical to those of the Earth (largely determined by the ambient temperature in the protoplanetary disk at that point, in turn dictated by the distance from the Sun). Under those conditions there is no need to explain the similar isotope ratios of Earth and Moon and this is thus not a constraint on the collision dynamics.

I recall the earth is slowing because of the tidal forces and the moon will eventually go into tidal lock. Can the slowing of rotation be calculated backwards to get an idea of length of a day back a few billion years ago?

The earth's rotation has been steadily decelerating. Since 1972 they've had to to add about 24 leap seconds to keep Coordinated Universal Time (UTC) accurate.

Because the earth itself is moving in orbit around the sun, the moon's sidereal period (27.3 days) and its synodic period (29.5 days) differ. The earth spins about 27 times faster than the moon moves around it. The gravitational pulls of the moon and sun to a smaller degree drag against the earth's surface and waters to create ocean tides. Although the Moon is much smaller, it exerts about 2½ times the pull on our ocean's tides as does the Sun which is 389 times farther away. During the periods of New Moon and Full Moon; both the Sun and the Moon are aligned and combine to exert a greater pull on the Earth's surface water, than at other times. Hi-tides will be observed on opposite sides of the globe and low tides observed in-between. Tides should not be thought of as strictly resultant of gravitational pulls, but as a pattern of resonance, having been set in motion over a long period of time.

We only see one side (a bit more than 50%) of the moon because it has slowed down to a synchronous rotation. In other words the moon rotates on its axis in about the same time that it takes to orbit the earth.

Maybe the colliding object was formed from the same dust that the earth and most of the earth's neighbors are. If that were the case wouldn't that explain why the isotope profiles look the same?

That's exactly what the mainstream "Theia formed at L4/5" concept is all about. L4/5 are at the same distance as the Earth from the Sun (they are the trojan points). Thus any body formed there would have the same composition as the Earth.

So, unless the impactor started right next door, we'd expect that the isotope ratios of the material it brought wouldn't look like the ones on Earth.

Um... the impactor (known as "Theia" -- also sometimes referred to as "Orpheus") is indeed supposed to have originated as our closest neighbor between the orbit of Earth and Mars, or possibly in an almost identical orbit as the Earth, kind of like the movie Journey to the far Side of the Sun.

Ok, here's what the paper says about this:"These would be consistent with prior simulations if the composition of the impactor’s mantle was comparable to that of the Earth’s mantle. It had been suggested that this similarity would be expected for a low-velocity impactor with an orbit similar to that of the Earth (4, 7, 8). However, recent work (9) finds this is improbable given the degree of radial mixing expected during the final stages of terrestrial planet formation"

The only problem with this is that anything as big as Mars probably originated from elsewhere in the Solar System, and thus would have a very distinct isotope ratio.

The other issue with this theory is that there would be an orbit gap someplace where a missing planet would be. The point is - where is the evidence of an additional planet ?

There were 1000's, at least, of rather large planetesimals flying around the Solar System in probably a whole variety of orbits. There's nothing 'missing'. Think of the early SS as very crowded. Over time bodies collided or interacted (some being ejected or thrown into the Sun) until only a few bodies remained in a set of mutually stable (actually semi-stable) orbits. It is unlikely that any extra planets could be added anywhere in the system (at least the inner system) and the whole thing remain stable over billions of years. In fact various simulations have been run with slight variations in the mass or orbital parameters of one or more inner planets. In no case is the result anywhere near as stable as what we have. In essence the early system tested a vast number of possible configurations and megayears of time filtered out all but one.

...the impactor (known as "Theia" -- also sometimes referred to as "Orpheus") is indeed supposed to have originated as our closest neighbor between the orbit of Earth and Mars, or possibly in an almost identical orbit as the Earth...

Ok, here's what the paper says about this:"These would be consistent with prior simulations if the composition of the impactor’s mantle was comparable to that of the Earth’s mantle. It had been suggested that this similarity would be expected for a low-velocity impactor with an orbit similar to that of the Earth (4, 7, 8). However, recent work (9) finds this is improbable given the degree of radial mixing expected during the final stages of terrestrial planet formation"

Thanks John and I believe that I understand what this article is describing --> A planet in the same orbit as Earth (such as the L4/L5 Lagrangian point) would impact face-on (perpendicular) and would not mix the elements properly.

I have a concern that a face-on impact would also not yield the Moon's nearly circular orbit because the debris would be flung sideways and might create a highly elliptical orbit as seen in my crude diagram. However, I don't have any tools to model this so I'm just speculating.

According to the youtube video narrated by Patrick Stewart that I linked to in my previous post, they believe the impactor hit on an oblique angle, originating from a position between the orbits of Earth and Mars. They do have Finite Element Analysis (FEA) computer models that simulate the oblique impact and show that the resulting Moon's orbit is correct and the mixing of the mantles and the cores is also correct.

The way to decide between these scenarios would likely become, if they don't make auxiliary predictions, to see how productive they are. Eg what scenario would most often result in the observed Earth and Moon masses and initial orbital parameters?

stevenkan wrote:

Why does that animation stop before the money shot?

At a guess because the simulation was specialized for the impact processes, not the accretion ones. You would use another simulation for that.

TheDS wrote:

Okay, so if two large objects with different isotope ratios merge into a single object, and then that single object splashes off some of itself to form another object, which has the same composition as the object it splashed from, why is this not computing?

You are assuming a perfect mixing of materials. This is the problem to achieve in other simulations.

random_name wrote:

"The only problem with this is that anything as big as Mars probably originated from elsewhere in the Solar System, and thus would have a very distinct isotope ratio."

But the classical Theia model (giant impact hypothesis) assumes that the (eventually Mars-sized) impactor started accumulating mass at Earth's lagrange point (L4 or L5), but its orbit became unstable around the time its mass grew big enough. Therefore, as it started from the same part of the disk around the sun, why would it need to have such a different isotope ratio?

These old models had to be abandoned, of course. They don't predict the required orbital parameters.

I had the most wonderful review, but the margin is too small to contain the link... Kidding, I can't find it and have run out of time. But if you dig after astronomer's reviewing the history of the field it ought to turn up eventually.

Short history as I remember it:

Theia initially never caught on. Resurfaced with the Apollo rocks showing Earth like isotope ratios. Revamped when it was realized a fast impactor, but not the original co-orbit Theia, would nicely predict the orbital parameters of Earth-Moon as well.

In other words, there may still be a "mainstream" L4/L5 theory building, but the mainstream Earth-Moon impact theory work is "not that" but on the more likely and better behaved usual disk planetesimals.